The present invention relates generally to the field of magnetic data storage and retrieval systems. More particularly, the present invention relates to a naturally differentiated magnetoresistive sensor.
In a magnetic data storage and retrieval system, a magnetic recording head typically includes a reader portion having a magnetoresistive (MR) sensor or reader for retrieving magnetically encoded information on a magnetic storage medium, such as a magnetic disc. Magnetic flux from the surface of the disc causes rotation of a magnetization vector of a sensing layer or layers of the MR sensor, which in turn causes a change in electrical resistive of the MR sensor. The sensing layers are often called “free” layers, because the magnetization vectors of the sensing layers rotate in response to external magnetic flux. A change in resistance of the MR sensor can be detected by passing a sense current, which is a fixed direct current (DC), through the MR sensor and measuring a DC voltage change across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the information encoded on the magnetic storage medium.
MR sensors have been developed that can be characterized in three general categories: (1) Anisotropic Magnetoresistive (AMR) sensors, (2) Giant Magnetoresistive (GMR) sensors, including Spin Valve sensors and multi-layer GMR sensors, and (3) Tunneling Magneto Resistive (TMR) sensors (also known as Tunneling Giant Magnetoresistive sensors).
AMR sensors generally have a single MR layer formed of a ferromagnetic material. The resistance of the MR layer varies as a function of Cos2 α, where α is the angle formed between the magnetization vector of the MR layer and the direction of the sense current flowing in the MR layer.
GMR sensors have a series of alternating magnetic and non-magnetic layers. The resistance of GMR sensors varies as a function of the spin-dependent transmission of conduction electrons between the magnetic layers separated by a non-magnetic layer and in the accompanying spin-dependent scattering, which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. The resistance of a GMR sensor depends upon the relative orientations of the magnetization vectors in consecutive magnetic layers, and varies as the cosine of the angle between the magnetization vectors of consecutive magnetic layers.
TMR sensors have a configuration similar to GMR sensors, except that the magnetic layers of the sensor are separated by an insulating non-magnetic film thin enough to allow electron tunneling between the magnetic layers. The tunneling probability of an electron incident on the barrier of one magnetic layer depends upon the character of the electrode wave function and the spin of the electron relative to the magnetization direction in the other magnetic layer. As a consequence, the resistance of the TMR sensor depends upon the relative orientations of the magnetization of the magnetic layers, exhibiting a minimum for a configuration in which the magnetizations of the magnetic layers are parallel, and at a maximum for a configuration in which the magnetizations of the magnetic layers are antiparallel.
For all types of MR sensors, magnetization rotation occurs in the sensing layers in response to magnetic flux from the magnetic storage medium (e.g., the magnetic disc). As the recording density of magnetic discs continues to increase, the width of the data tracks on the discs must decrease, which necessitates correspondingly smaller and smaller MR sensors. As MR sensors become smaller in size, particularly for sensors with dimensions less than about 0.1 micrometers (μm), the sensors have the potential to exhibit an undesirable magnetic response to applied fields from the magnetic disc. MR sensors must be designed in such a manner that even small sensors provide a signal with adequate voltage amplitude and minimal noise interference (e.g., media noise and electronic noise). This requires the signal-to-noise (SNR) ratio to be sufficiently high for accurate recovery of the data on the disc.
With longitudinal recording, the magnetic medium includes a plurality of bits, each bit having a magnetization direction arranged parallel to an air bearing surface (ABS) of the transducing head. In traditional longitudinal recording, a generally bell-shaped waveform is generated as the longitudinal reader crosses a single transition on the magnetic medium (i.e., where magnetization of the bits changes polarity). This bell-shaped curve has a minimum voltage (V0) when the reader is positioned over a portion of the magnetic medium having substantially constant magnetization, and has a relative maximum or peak voltage (V1) as the reader crosses a transition. Readers detect variations in magnitude of the playback voltage for reading data from the magnetic medium.
With perpendicular recording, magnetization directions of individual bits on the magnetic medium are arranged orthogonal to an air bearing surface of the transducing head. With traditional perpendicular recording (i.e., non-differentiated perpendicular recording), a playback waveform is generated such that playback voltage has a magnitude of approximately zero when the reader crosses a transition on the magnetic medium, and reaches a positive or negative maximum voltage (V1) when the transducing head is positioned over a region of the magnetic medium having a substantially constant magnetization direction. The playback waveform generated with traditional perpendicular recording is not bell-shaped. In order to produce a bell-shaped playback waveform similar to that produced with longitudinal recording, differentiated readers are required for perpendicular recording.
Generally, a differentiated reader is defined as a reader that dynamically detects a difference in magnetization directions of magnetic layers having magnetization directions capable of some rotation, by measuring a change in resistance of the reader. Differentiated readers typically include two separate reader or sensor elements physically separated by a gap film (i.e., a film located in a reader gap that separates other layers). A reader element is a component, typically comprised of a plurality of layers, generally capable of producing a MR or GMR effect for reading magnetically stored data. The equilibrium magnetization directions of the respective free layers of each reader element are typically influenced by an external magnetic field in a quiescent state. The two reader elements are typically arranged such that one reader element encounters a transition on a corresponding magnetic medium before the other reader element. A generally bell-shaped playback waveform is achieved by adding (or subtracting) the signals from both the reader elements of the differentiated reader. In that way, differentiated readers detect field variations for reading data from the magnetic medium. However, known differentiated perpendicular transducing heads produce a playback waveform that has a smaller amplitude than playback waveforms typical of longitudinal recording.
Differentiated readers exhibit a significant reduction in playback voltage as compared to longitudinal systems. For example, the zero-to-peak voltage change in the playback waveform for longitudinal recording is about 1-2 microvolts (μV). In contrast, known differentiated reader systems typically have a playback voltage of about one tenth ( 1/10) or less of the magnitude of known longitudinal playback systems. This is problematic in that it is desired to achieve a relatively high signal-to-noise ratio (SNR) with the playback signal. For example, assuming noise remains constant, a 50% reduction in playback voltage amplitude corresponds to reduction in the SNR of about 6 decibels (dB).
Another problem with known designs is that due to a voltage dropoff as the reader is positioned over interior portions of a large DC region of a magnetic medium (i.e., a region having a constant magnetization direction), read errors may occur. Such read errors occur when the reader mistakes decreased voltage in the DC region for a transition (i.e., a change in polarity of the magnetization of bits on the magnetic medium).
Both theory and simulation show that to achieve the same playback amplitude in differentiated perpendicular heads as compared to longitudinal heads, the differentiated perpendicular designs with two or more separate read elements must have sensor spacing equal to or larger than the pulse width at half maximum (PW50) for the playback waveform. PW50 is given by the following equation, where “g” represents reader shield-to-shield spacing, “d” represents fly height or head-to-media separation, “a” represents a transition parameter, and “δ” represents media thickness:
      PW    50    =                    g        2            +              4        ⁢                  (                      d            +            a                    )                ⁢                  (                      d            +            a            +            δ                    )                    
However, because PW50 is always larger than shield-to-shield spacing, it is not possible to design a differentiated reader with two separate reader elements and adequate playback amplitude.
Because of the high costs associated with specialized circuitry for performing differentiation calculations, it is desirable to use circuitry common in the art for interpreting signals from an MR sensor.
Thus, the present invention relates to a naturally differentiated reader for perpendicular transducing heads having a playback waveform with an amplitude comparable to that for longitudinal recording.